fluid

1. Thermal conductivity, also called heat conduction, is the flow of energy from
something of a higher temperature to something of a lower temperature. It is different
from electrical conductivity, which deals with electrical currents. Several factors affect
thermal conductivity and the rate that energy is transferred. As the Physics Info
website points out, the flow is not measured by how much energy is transferred, but
by the rate it is transferred.
Material
The kind of material being used in thermal conductivity can affect the rate of energy
flowing between the two regions. The greater the conductivity of the material, the
faster the energy flows. According to the Physics Hypertextbook, the material with the
greatest conductivity is helium II, a superfluid form of liquid helium, which exists only
at very low temperatures. Other materials with high conductivity are diamonds,
graphite, silver, copper and gold. Liquids have low conductivity levels and gases even
lower.
Length
The length of the material the energy must flow through can affect the rate at which it
flows. The shorter the length, the faster it will flow. The thermal conductivity may
continue to increase even when the length is increased--it may just increase at a
slower pace than it had before.
Termperature Difference
Thermal conductivity varies depending on temperature. Depending on the material of
the conductor, as the temperature rises the thermal conductivity of the material often
rises as well, increasing the flow of energy.
Cross-Section Types
The cross-section type, such as round, C- and hollow-shaped, can affect the thermal
conductivity, according to the Journal of Materials Science. The article reports that the
thermal diffusivity factor of C- and hollow-shaped carbon fiber-reinforced composites
showed about two times higher values than those of round-type ones.